Link Quality Estimation and Apparatus in a Telecommunication System

ABSTRACT

Method and apparatus for enabling accurate link quality estimation of a wireless link between a sending node and a receiving node. When the sending node receives link state reports from the receiving node, it estimates the current state of the wireless link. The sending node also determines a measurement adjusting parameter if the link state reports are deemed inaccurate in relation to the estimated link state, based on a deviation between the received link state reports and the estimated actual link state. The sending node then sends the determined measurement adjusting parameter to the receiving node, and the receiving node provides a link state report based on signal measurements adjusted by the measurement adjusting parameter. The adjusted link state report can then be used for link adaptation of the wireless link and/or for packet scheduling decisions.

This application is a continuation of U.S. application Ser. No.13/660,158, filed 25 Oct. 2012, which is a continuation of U.S.application Ser. No. 12/866,585, filed 6 Aug. 2010, which was theNational Stage of International Application No. PCT/EP2008/058217, filed26 Jun. 2008, which claims benefit of U.S. Provisional Application No.61/027,535 filed 11 Feb. 2008, the disclosures of each of which areincorporated herein by reference in their entirety.

TECHNICAL FIELD

The present invention relates generally to a method and apparatus foroptimizing wireless transmissions in a telecommunication system by meansof more accurate link quality estimation.

BACKGROUND

In 3GPP (3^(rd) Generation Partnership Project), the packet-switchedcommunication systems HSPA (High Speed Packet Access) and LTE (Long TermEvolution) have been specified for wireless transmission of data packetsbetween user terminals and base stations in a cellular/mobile network.In this description, the term “base station” is used to generallyrepresent any network node capable of wireless communication with a userterminal.

LTE systems generally use OFDM (Orthogonal Frequency DivisionMultiplexing) involving multiple narrow-band sub-carriers which arefurther divided into time slots to form a so-called “time-frequencygrid” where each frequency/timeslot combination is referred to as a“Resource Element RE”. In LTE, multiple antennas can also be employed inboth user terminals and base stations for obtaining parallel andspatially multiplexed data streams, e.g. according to MIMO (MultipleInput Multiple Output), which is well-known in the art. Other wirelesscommunication systems relevant for the following description includeWCDMA (Wideband Code Division Multiple Access), WiMAX, UMB (Ultra MobileBroadband), GPRS (General Packet Radio Service) and GSM (Global Systemfor Mobile communications).

A base station of a cell in a wireless network may transmit data andcontrol information in a physical downlink channel to a user terminal or“UE” (User Equipment), and a user terminal may likewise transmit dataand control information in a physical uplink channel in the oppositedirection to the base station. In this description, a physical downlinkor uplink channel is generally referred to as a wireless link between asending node and a receiving node. Further, the terms “sending node” and“receiving node” are used here merely to imply the direction of thewireless link considered, although these nodes can of course bothreceive and send data and messages in an ongoing communication. Further,the term “Resource Element RE” is used in this description to generallyrepresent a signal bearer element that can carry a signal over awireless link, without limitation to any transmission technology such asLTE. For example, an RE can incorporate a specific code and timeslot ina system using CDMA (Code Division Multiple Access), or a specificfrequency and timeslot in a system using TDMA (Time Division MultipleAccess), and so forth.

When two nodes in a cell communicate over a wireless link that isconfigured according to various link parameters, one or more such linkparameters can be adapted to the current state of the link on a dynamicbasis, often referred to as link adaptation. Such link parameters mayinclude transmission power, modulation schemes, encoding schemes,multiplexing schemes, and the number of parallel data streams whenmultiple antennas are used, the latter link parameter being called“transmission rank”. Link adaptation is used to generally optimizetransmission in order to increase capacity and data throughput in thenetwork. Further, link adaptation can be employed for the uplink and thedownlink independently, if applicable, since the current state of theuplink and downlink can be very different, e.g. due to differentinterference and when frequency and/or time are widely separated foruplink and downlink transmissions between the two nodes.

To support link adaptation during an ongoing communication between asending node and a receiving node, either on the uplink or downlink, thereceiving node is often required to measure certain link parameters andreport recommended link parameters to the sending node, such as arecommended transmission rank and/or a recommended precoder matrix.Also, the quality of the received signal is often measured, typically interms of a Signal to Interference and Noise Ratio SINR, e.g. separatelyfor different parallel data streams, assuming that the recommended linkparameters are used by the sending node. Based on the recommended linkparameters and measured SINR value(s), the receiving node estimatesso-called “Channel Quality Indicators” CQIs, e.g. one CQI for each codeddata block (codeword), that are used together with the link parametersto indicate the current state of the link, which is reported back to thesending node. In this description, a reported CQI or the equivalentand/or recommended link parameters will be called a “link state report”for short. The sending node can then adapt one or more link parametersdepending on the received link state report. When the sending node is abase station using packet switching for downlink transmissions, thereported CQIs may also be used for packet scheduling decisions.

Typically, specific known reference symbols RS are regularly transmittedover a wireless link according to a predetermined scheme to support theabove link quality estimation, such that the receiving node is able todetect noise and interference more easily without having to decode thereceived signal. In an OFDM-based LTE system, these RSs are transmittedfrom base stations in predetermined REs in the time-frequency grid asknown by the receiving terminal.

In general, a received signal “r” in an RE is basically comprised oftransmitted symbols “s” as well as noise and interference “n”. Thus:

r=Hs+n  (1)

Generally, r, s and n are vectors and H is a matrix, where “H”represents the channel response which can be derived from a channelestimator in the receiver. However, the noise and interference of asignal in an RE display different characteristics depending on whetherthe RE contains payload data, control signalling or an RS, as theinterference mix hitting the different types of REs may typically havedifferent transmission power and spatial characteristics, e.g. due totime and/or frequency synchronization in neighboring cells. Theinterference/noise “I” in these different signal types may becharacterized in terms of second order statistics that can be obtainedby frequently measuring the signals over time, although “I” can becharacterized in other ways as well.

If an RE contains an RS signal received by a user terminal, the terminalis able to estimate the interference/noise n=I(RS) of the RS signalsince s are known symbols in this case and H is given by the channelestimator. If the RE contains data scheduled for the terminal, theinterference/noise n=I(data) can also be estimated once the data symbolshave been detected (i.e. decoded) by the terminal, s thereby being knownat that point. Similarly, the interference/noise of an RE with controlsignalling, n=I(control), can be estimated if the control symbols can bedetected.

In order to obtain proper link quality estimation and to determine anaccurate CQI and/or link parameter recommendation for a link, thereceiving node needs sufficient statistics from measuring signalstransmitted on the link. Furthermore, the characteristics of inter-cellinterference may be significantly different depending on what signaltype is causing the interference from neighboring cells, i.e. RSsignals, data signals or control signals. If payload data is transmittedover the link to be estimated, the receiving node should preferablymeasure the interference I(data) that hits the data signals. However,the measurements would then be limited to REs that contain datascheduled for the user terminal involved, which may be too scarce suchthat the statistic basis for determining the CQI is insufficient.Moreover, the data symbols must be detected and decoded, and possiblyalso re-encoded, before the interference I(data) can be properlyestimated, which may impose substantial costs and/or unacceptable delaysdue to the data processing.

Alternatively or additionally, the receiving node can measure theinterference I(RS) for REs containing an RS which may occur morefrequently than the REs containing scheduled data. Measuring I(RS) isalso generally more reliable since the RS is always known to thereceiving node. However, the interference that hits RS signals may besignificantly different from that hitting the data signals, e.g. withrespect to statistics. Therefore, a CQI and/or link parameterrecommendation determined from I(RS) measurements may not berepresentative for a link with payload data transmission. As a result,the link adaptation at the sending node may not be optimal for data dueto either too optimistic or too pessimistic CQI and/or link parameterrecommendation from the receiving node. Hence, if the measured I(RS) issignificantly greater than the actual I(data), the CQI and/or linkparameter recommendation will be based on an overestimated interference(or underestimated SINR) and therefore unduly pessimistic, and viceversa.

For example, when MIMO is employed in an LTE system, the RE holding anRS from one antenna at the sending node must be empty for a neighboringantenna, which substantially limits the number of REs available for RStransmissions. As a result, the interference that hits REs containing anRS will largely come from RS transmissions in other cells due to reuseof the RS transmission pattern. As mentioned above, RSs are alwaystransmitted from base stations according to a predetermined scheme andat a relatively high fixed power in order to be received by any terminalin the cell, whereas payload data is only transmitted when scheduled fora specific terminal. Thus, in a situation with low data traffic and/orlow transmission power for data signals, I(data) is generally lower thanI(RS).

Furthermore, control signals are often transmitted with greater powerthan data signals, due to different power regulation. Therefore, theinterference measured for an RE affected by control signal interferencemay be different from that of an RE affected by data signalinterference.

Hence, it is often difficult to obtain accurate estimates of theinter-cell interference that hits data transmissions, in particular ifthe interference measurements are performed on RS transmissions, asexplained above. Inaccurate estimates of the SINR may thus result inmisleading CQIs and non-optimal link parameter recommendations such astransmission rank. A consequence for MIMO systems is that anunderestimated SINR may result in a too pessimistic transmission rankwhen the used link can actually support a transmission rank greater thanthe recommended one. Both of these issues may well result in reducedthroughput. On the other hand, if the SINR is overestimated, the linkmay not be able to support any recommended CQIs (including a recommendedModulation and Coding Scheme MCS) and transmission rank, resulting inexcessive decoding errors and thereby reduced throughput also in thiscase.

However, the base station may monitor so-called “ACK/NACK signalling”from the terminal for received data blocks, and detect if a Block ErrorRate BLER or the like is below or above a predetermined target value.From this information, the base station can decide to use a moreoffensive or defensive MCS than recommended by the terminal. However, ifthe base station selects a transmission rank different from therecommended one, the reported CQI will be largely irrelevant since, inmost cases, it relates directly to the transmission rank. Consequently,the base station would not have a proper basis for selecting the MCS andother link parameters for the different data streams.

It is thus generally a problem that, in a communication with dynamiclink adaptation, a signal sending node may receive inaccurate linkquality estimations and/or link parameter recommendations from a signalreceiving node, such that the used link parameters are not optimal orappropriate for the actual link used in the communication.

SUMMARY

It is an object of the present invention to generally address theproblems outlined above. Further, it is an object to provide a solutionfor obtaining more accurate link or channel quality estimation and/ortransmission rank recommendations, e.g. to support dynamic linkadaptation of a wireless link. These objects and others may beaccomplished by a method and apparatus according to the attachedindependent claims.

According to one aspect, a method is provided in a sending node forenabling accurate link quality estimation of a wireless link used fortransmitting signals from the sending node to a receiving node. In themethod, at least one link state report is received from the receivingnode, and the current state of the wireless link is also estimated. Ameasurement adjusting parameter is determined if the at least onereceived link state report is deemed inaccurate in relation to theestimated link state, based on a deviation between the received linkstate report(s) and the estimated actual link state. The determinedmeasurement adjusting parameter is then sent to the receiving node, anda link state report is received from the receiving node which is basedon signal measurements adjusted by the measurement adjusting parameter.Thereby, inaccurate link quality estimations and/or link parameterrecommendations can be avoided, and the sending node is able to useoptimal or appropriate link parameters when communicating with thereceiving node.

According to another aspect, an apparatus is provided in a sending nodefor enabling accurate link quality estimation of a wireless link usedfor transmitting signals from the sending node to a receiving node. Thesending node apparatus comprises a sending unit adapted to send signalsto the receiving node over the wireless link, a report receiver adaptedto receive link state reports from the receiving node, and a link stateestimator adapted to estimate the current state of the wireless link.The sending node apparatus further comprises a determining unit adaptedto determine a measurement adjusting parameter if at least one receivedlink state report is deemed inaccurate in relation to the estimated linkstate, based on a deviation between the received link state report(s)and the estimated link state, and to send the determined measurementadjusting parameter to the receiving node. The report receiver isfurther adapted to receive a link state report from the receiving nodewhich is based on signal measurements adjusted by the measurementadjusting parameter.

Different embodiments are possible in the sending node method andapparatus above. In one exemplary embodiment, the sending unit uses theadjusted link state report for link adaptation of the wireless linkand/or for packet scheduling decisions. In another exemplary embodiment,the sending unit sends payload data and reference symbols to thereceiving node which configures the link state reports based on signalmeasurements on the reference symbols, where the measurement adjustingparameter compensates for a difference in received power or SINR betweenmeasured signals and data signals.

The measurement adjusting parameter may be a Power Measurement OffsetPMO that the receiving node uses for adjusting signal power or SINRmeasurements upon which the adjusted link state report is based.

Further, the link state reports may comprise a link quality estimationand/or link parameter recommendation, where the link quality estimationmay comprise a Channel Quality Indicator CQI. The link parameterrecommendation may comprise a preferred transmission rank specifying thenumber of parallel data streams when multiple antennas are used.

According to further exemplary embodiments, the link state estimator mayestimate the current state of the wireless link by monitoring the amountof data errors occurring over the wireless link as compared to apredetermined target value. The link state estimator may then monitorACK/NACK messages from the receiving node to determine whether a BlockError Rate BLER or equivalent parameter deviates from the target value.The link state estimator may also estimate the current state of thewireless link by monitoring the current traffic load in the networkused.

According to yet another aspect, a method is provided in a receivingnode for enabling accurate link quality estimation of a wireless linkused for transmitting signals from a sending node to the receiving node.In this method, at least one link state report is sent to the sendingnode containing a link quality estimation and/or link parameterrecommendation. When a measurement adjusting parameter is received fromthe sending node, a link quality estimation and/or link parameterrecommendation is/are determined based on signal measurements adjustedby the received measurement adjusting parameter. An adjusted statereport is then sent to the sending node containing the determined linkquality estimation and/or link parameter recommendation.

According to yet another aspect, an apparatus is provided in a receivingnode for enabling accurate link quality estimation of a wireless linkused for transmitting signals from a sending node to the receiving node.This apparatus comprises a signal receiving unit adapted to receivesignals from the sending node over the wireless link, a signal measuringunit adapted to measure received signals, a quality estimating unitadapted to estimate link quality and/or determine recommended linkparameters, and a reporting unit adapted to send link state reports tothe sending node. The quality estimating unit is further adapted toobtain a measurement adjusting parameter from the sending node, and todetermine a link quality estimation and/or link parameter recommendationbased on signal measurements adjusted by the received measurementadjusting parameter. The reporting unit is further adapted to send anadjusted link state report to the sending node containing the determinedlink quality estimation and/or link parameter recommendation.

Different embodiments are possible in the receiving node method andapparatus above. In one exemplary embodiment, the signal receiving unitreceives payload data and reference symbols from the sending node, andthe reporting unit configures the link state reports based on signalmeasurements on the reference symbols, where the measurement adjustingparameter compensates for a difference in received power or SINR ofmeasured signals and data signals.

The measurement adjusting parameter may be a Power Measurement OffsetPMO that is used for adjusting signal power or SINR measurements uponwhich the adjusted link state report is based.

Further possible features and benefits of the present invention will beexplained in the detailed description below.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in more detail by means of exemplaryembodiments and with reference to the accompanying drawings, in which:

FIG. 1 is a flow chart illustrating a procedure for enabling accuratelink quality estimation as performed by a signal sending node, inaccordance with one embodiment.

FIG. 2 is a flow chart illustrating a procedure for enabling accuratelink quality estimation as performed by a signal receiving node, inaccordance with another embodiment.

FIG. 3 is a signal power diagram illustrating different power levelswhen the present invention is used for link quality estimation, inaccordance with yet another embodiment.

FIG. 4 is a block diagram illustrating a signal sending node and asignal receiving node in more detail, in accordance with furtherembodiments.

DETAILED DESCRIPTION

The present invention can be used to avoid inaccurate link qualityestimations and/or link parameter recommendations, such that a sendingnode is able to use optimal or appropriate link parameters whentransmitting payload data in communication with a receiving node. Inparticular, more accurate transmission rank recommendations can beobtained such that a sending node can utilize recommended CQIs to alarge extent, because it does not have to override the recommendedtransmission rank. In the following description, it is assumed that linkadaptation based on link state reports is employed, although the presentinvention is generally not limited thereto. In addition oralternatively, accurate link quality estimation can further be usefulfor scheduling decisions in packet-switched communications.

Briefly described, the sending node determines whether the reportingfrom the receiving node with link quality estimation and/or linkparameter recommendation is accurate or inaccurate for the actual linkused, by estimating the current state of the link. The link state can beestimated in different ways, e.g. by monitoring ACK/NACK messages fromthe receiving node to see how much data errors occur in thetransmission, and/or by monitoring the current traffic situation in thenetwork, which will be described in more detail below.

If the estimated link state indicates that the link state report isinaccurate, the sending node determines a “Power Measurement Offset PMO”or other measurement adjusting parameter that the receiving node willuse for adjusting the signal power or SINR measurements or other signalmeasurements upon which the link quality estimation and/or linkparameter recommendation is based. The receiving node then sends aPMO-adjusted link state report to the sending node which is able to usethe PMO-adjusted link state report for more appropriate link adaptation.Thereby, link parameters will be selected that are more closely adaptedto the current link state and with consideration to what the link canactually support.

The sending node may strive to configure a PMO profile such that theresulting link state reports from the receiving node becomes relevant oraccurate for the estimated link state, e.g. by employing an iterativeprocess of testing different PMO profiles. The sending node may alsostrive to configure the PMO profile such that the amount of data errorsin the transmission does not significantly deviate from a target value.In general terms, the PMO is thus effectively a “measurement adjustingparameter”, and these two expressions can be used in this descriptioninterchangeably.

FIG. 1 is a flow chart illustrating an exemplary procedure for enablingappropriate link quality estimation, as performed by a signal sendingnode in communication with a signal receiving node over a wireless link.The sending node may be a base station or the like and the receivingnode may be a user terminal, or vice versa, and it should be noted thatthe terms sending node and receiving node merely indicate the directionof the link under consideration. In a first step 100, a regular linkstate report is received from the receiving node containing a linkquality estimation and/or a link parameter recommendation. The receivingnode has thus made a link quality estimation in a more or lessconventional manner based on signal measurements, e.g. measurements ofsignal power or SINR on received RSs as described above, which isreflected in the link state report.

In a next step 102, the sending node estimates the current state of thelink, which can be made in different ways. For example, whenretransmission of data blocks based on ACK/NACK reports is employed in aHARQ (Hybrid Automatic Repeat ReQuest) process to correct anyerroneously received data, the ACK/NACK messages from the receiving nodemay be monitored to determine whether the Block Error Rate BLER orsimilar parameter deviates from a predetermined target value. If theBLER is below the target value, it is assumed that the receiving nodehas underestimated the link quality in the link state report, and viceversa. ACK/NACK messages from other nodes may also be taken into accountwhen the sending node estimates the link state. However, the amount oferrors can be monitoring in other ways, depending on the technology andprotocols used. Further, the current traffic load in the network mayalso be monitored, assuming that a high load in the area from ongoingdata transmissions generally results in relatively high interference,and vice versa.

It is then determined in a following step 104 whether the estimated linkstate indicates that the received link state report is inaccurate, i.e.misleading and not reflecting the true link state or quality. Asdescribed above, this may be the case when the receiving node measuresthe signal power or SINR for REs containing RSs instead of payload dataand when the interference from data transmissions is relatively low,resulting in a report with underestimation of the link quality withrespect to data transmissions.

If the received link state report is determined to be accurate bymatching the estimated link state, it can be used for relevant andappropriate link adaptation in an optional step 106 and/or forscheduling decisions for packet-switched communications. However, if thelink state report is deemed inaccurate in relation to the estimated linkstate, a measurement adjusting parameter or PMO profile is determinedbased on the deviation between the received link state report and theestimated actual link state, which is sent to the receiving node, in afurther step 108.

The measurement adjusting parameter or PMO profile may be conveyed tothe receiving node by means of suitable control signalling such ascommon control signalling, e.g. broadcast, or dedicated controlsignalling, e.g. RRC (Radio Resource Control). It will be described inmore detail later below how a PMO profile can be determined by thesending node and used by the receiving node in the case when the signalpower in a measured channel deviates from that of a data channel, e.g.when REs containing RSs are being measured.

The receiving node will now use the measurement adjusting parameter orPMO profile for adjusting the signal measurements, e.g. signal power orSINR, upon which the link quality estimation and/or link parameterrecommendation is based, to compensate for any underestimation oroverestimation of the signal power or SINR or other measured parameter.A PMO-adjusted link state report is then received from the receivingnode in a next step 110, containing a link quality estimation and/or alink parameter recommendation based on signal measurements, e.g. signalpower or SINR, adjusted by the measurement adjusting parameter or PMOprofile determined and sent in step 108.

The sending node is now able to use the PMO-adjusted link state reportfor obtaining a more appropriate link adaptation, in an optional finalstep 112. Alternatively or additionally, the PMO-adjusted link statereport can also be used for scheduling decisions for packet-switchedcommunications, as similar to step 106 above. When receiving aPMO-adjusted link state report from the receiving node in step 110, thesending node may iteratively repeat steps 104, 106 and 110, as shown bythe dashed arrow, to find out if the used PMO profile was apt.Meanwhile, link adaptation may be employed according to step 112 usingthe latest received PMO-adjusted link state report. Furthermore, thestep 102 of estimating the link state may be executed on a more or lesscontinuous basis in order to keep the typically fluctuating link stateup-to-date.

In this way, more accurate link state reports can be obtained from thereceiving node and more appropriate link adaptation and/or schedulingdecisions can therefore also be made based on the link state reports.For example, when MIMO is used in LTE for multiple data streams, movingthe compensation of erroneously estimated interference from the basestation to the user terminal may significantly improve the accuracy ofthe recommended transmission rank and thereby also the accuracy of thereported CQI which relates directly to the transmission rank. Thisprocedure thus provides a mechanism for the base station to alter the“aggressiveness” or “defensiveness” of the CQI estimation in the userterminal by incorporating the transmission rank preferred by the userterminal.

The flow chart of FIG. 2 illustrates an exemplary procedure for enablingappropriate link quality estimation, as performed by a signal receivingnode in communication with a signal sending node over a wireless link,where the sending node basically executes the procedure of FIG. 1. In afirst step 200, a CQI and/or link parameter recommendation is determinedin a more or less conventional manner based on signal measurements, e.g.signal power or SINR measurements, and a resulting link state report issent to the sending node, as corresponding to step 100. In a next step202, a measurement adjusting parameter or PMO profile is received fromthe sending node, e.g. by means of common or dedicated controlsignalling, as a consequence of detecting that the previous link statereport did not match the actual link state, as corresponding to steps104 and 108.

In a following step 204, the receiving node determines a link qualityestimation, e.g. CQI, and/or link parameter recommendation based onsignal measurements such as signal power or SINR adjusted by thereceived measurement adjusting parameter or PMO. The adjustment ofsignal power or SINR measurements will be described in more detail belowwith reference to FIG. 3. Finally, a PMO-adjusted link state report issent to the sending node in a last shown step 206, containing the abovelink quality estimation and/or a link parameter recommendation. Thesending node will then be able to use the PMO-adjusted link state reportfor obtaining a more appropriate link adaptation, as of step 112.

FIG. 3 is a schematic diagram illustrating how the receiving node canadjust a measured signal power or SINR with a PMO received from thesending node. The vertical arrow in the figure represents a power scaleon which different power levels are shown as horizontal lines 300-304a/b. In this case, a logical channel on the wireless link is measuredwhere the signal power or SINR deviates from that of a logical datachannel which is used for transmission of payload data. In this example,the measured channel contains RSs with different interference ascompared to the current interference on the data channel.

First, the receiving node measures the signal power or SINR 300 of themeasured channel. Further, a predetermined and stipulated power offsetvalue between data and RS has been provided, e.g. from the sending node,which the receiving node uses to compensate for a typically occurringdifference in the power between data and RS. Thereby, a compensatedpower/SINR level 302 is obtained and the receiving node determines alink quality estimation and/or link parameter recommendation based onthe compensated power/SINR level 302 and sends a corresponding linkstate report to the sending node. Preferably, the receiving nodeprovides a plurality of such link state reports to the sending node toprovide sufficient statistics and basis for the sending node to assessthe reports.

Next, the sending node determines a PMO profile after detecting that thelink state report does not match the actual link state, as describedabove, and sends the PMO profile to the receiving node. Generally, a PMOprofile may comprise one or more specific measurement adjustingparameters depending on the implementation. As described above, theactual link state can be estimated by monitoring ACK/NACK messages,which should preferably be made basically at the same time as the linkstate reports are made.

The receiving node then adjusts the power/SINR level 302 by the PMO andobtains a PMO-adjusted power/SINR level 304 a, in this case being ahigher and thus more “optimistic” power/SINR level than level 302 due toan underestimated Power/SINR. In another example, the PMO-adjustedpower/SINR level 304 b, may be lower and thus a more “pessimistic”power/SINR level than level 302 due to an overestimated Power/SINR, asindicated by the dashed lines.

Some more detailed examples will now be described of how the sendingnode can calculate a PMO profile and how the PMO can be used by thereceiving node to compute a CQI and recommended link parameters such astransmission rank. Using the formula (1) above, the data channel can bemodeled as:

r(data)=H(data)s+n(data)  (2)

As mentioned above, the measured channel may deviate from the datachannel and the measurement channel can be denoted as:

r(m)=H(m)s+n(m)  (3)

The PMO profile, configured by the sending node, effectively describesthe mapping from the measurement channel to the data channel. Forexample, the PMO profile could be a channel power scaling value“P(PMO)”. In order to determine CQI and preferred transmission rank, thedata channel can be estimated as:

r(data)≈sqrtP(PMO)H(m)s+n(m)  (4)

where “sqrtP(PMO)” denotes the square root of P(PMO).

Other ways of estimating the data channel are also conceivable, as forexample:

r(data)≈H(m)s+sqrtP(PMO)n(m)  (5)

or:

r(data))≈H(m)s+n(m)+sqrtP(PMO)I(m)  (6)

In (6), the noise and interference of the measured channel have beendivided into a separate noise term n(m) and a separate interference termI(m).

When the receiving node computes the CQI and recommended link parameterssuch as transmission rank, it uses the estimated data channel instead ofusing the measurement channel. It should be noted that the noise andinterference are modeled statistically and may also be averaged overtime and frequency, to capture long term variations rather than a shortterm behavior.

The signal sending node may determine a suitable PMO profile based onseveral possible input variables. For example, the ACK/NACK signallingof a HARQ process can be monitored. If the BLER or a similar parameterreflecting the data error rate does not match a target value, the PMOprofile is adjusted accordingly. If the PMO profile of (4) is used andthe BLER is detected to be below the target value, the P(PMO) isincreased and the receiving node will use a more optimistic mapping fromthe measurement channel to the data channel when determining CQI andrecommended link parameters, e.g. transmission rank. This may beaccomplished by using a PMO value to adjust an assumed RS/data offset,as shown in FIG. 3, to a more realistic power value considering theactual link state.

As mentioned above, a base station acting as the sending node could alsodetermine the PMO profile based on HARQ ACK/NACK statistics frommultiple user terminals in the area, possibly filtered over a period oftime. Alternatively or additionally, the PMO profile could be determinedbased on the current traffic load in the network, e.g. the average loadin surrounding cells. A base station is able to obtain such traffic loadinformation by means of so-called backhaul signalling, e.g. according tothe X2 protocol.

A signal sending node and a signal receiving node will now be describedin more detail with reference to FIG. 4, in accordance with furtherexemplary embodiments. The signal sending node 400, which may be a basestation, is basically configured to perform the steps in FIG. 1, whereasthe signal receiving node 402, which may be a user terminal, isbasically configured to perform the steps in FIG. 2. In the sending node400, a sending unit 400 a is configured to send at least data signalsand RS signals to the receiving node 402 over a wireless link underconsideration.

Receiving node 402 comprises a receiving unit 402 a configured toreceive the data and RS signals, a signal measuring unit 402 b adaptedto measure received signals, e.g. with respect to power or SINR, aquality estimating unit 402 c adapted to estimate link quality, e.g.CQI, and determine recommended link parameters, and a reporting unit 402d adapted to send link state reports to the sending node 400.

Sending node 400 further comprises a report receiver 400 b adapted toreceive link state reports from node 400, and a link state estimator 400c adapted to estimate the current state of the considered link, e.g.based on received information on the amount of data errors occurring onthe link and/or information on the current traffic load in the network,as indicated by a dashed arrow. Sending node 400 also comprises adetermining unit 400 d adapted to determine a measurement adjustingparameter, e.g. a PMO profile, if a link state report received from node402 does not match the estimated link state, based on the estimated linkstate in relation to the link state report, and further adapted toprovide the measurement adjusting parameter to the quality estimatingunit 402 c in node 402.

Quality estimating unit 402 c is further adapted to obtain a measurementadjusting parameter from the sending node 400, and to determine a linkquality estimation and/or link parameter recommendation based on signalmeasurements, e.g. signal power or SINR measurements, adjusted by thereceived measurement adjusting parameter. Reporting unit 402 d isfurther adapted to send an adjusted link state report to the sendingnode 400 containing the determined link quality estimation and/or linkparameter recommendation. The report receiver 400 b is further adaptedto receive the adjusted link state report from the receiving node 402which is based on signal measurements adjusted by means of themeasurement adjusting parameter. Finally, the sending unit 400 a may befurther adapted to use the adjusted link state report for linkadaptation of the wireless link.

It should be noted that FIG. 4 merely illustrates various functionalunits in the sending and receiving nodes 400, 402 in a logical sense,while these functions can be implemented in practice using any suitablesoftware and hardware, without departing from the present invention.

By using any of the above-described embodiments, more accurate linkadaptation, power control and/or scheduling may be accomplished,potentially resulting in improved capacity, coverage and/or quality inthe network. Any estimation errors of noise and inter-cell interferencemay thus be compensated, such that the link state reports from thereceiving node, e.g. including estimated CQI and recommendedtransmission rank, will match and be closely aligned with what thechannel actually supports.

While the invention has been described with reference to specificexemplary embodiments, the description is in general only intended toillustrate the inventive concept and should not be taken as limiting thescope of the invention. For example, although the concepts of LTE, OFDM,MIMO, CQI, SINR, resource elements, transmission rank, HARQ and ACK/NACKmessages have been used when describing the above embodiments, any othersimilar suitable standards, parameters and mechanisms may basically beused to accomplish the functions described herein. The present inventionis generally defined by the following independent claims.

What is claimed is:
 1. A method, in a sending node, of enabling linkquality estimation of a wireless link used for transmitting signals fromthe sending node to a receiving node, the method comprising: receivingat least one link state report; estimating the current state of thewireless link; sending a measurement adjusting parameter to thereceiving node; and receiving a subsequent link state report containinga determined link quality estimation or a link parameter recommendation,or both, which are based on signal measurements adjusted by themeasurement adjusting parameter.